Dementia is a clinical syndrome characterized by deficits in multiple areas of cognition that cannot be explained by normal aging, a noticeable decline in function, and an absence of delirium. In addition, neuropsychiatric symptoms and focal neurological findings are usually present. Dementia is further classified based on etiology Alzheimer's disease (AD) is the most common cause of dementia, followed by mixed AD and vascular dementia, vascular dementia, Lewy body dementia (DLB), and fronto-temporal dementia.
β-Amyloid deposits and neurofibrillary tangles are considered to be major pathologic characterizations associated with AD which is characterized by the loss of memory, cognition, reasoning, judgment, and orientation. Also affected, as the disease progresses, are motor, sensory and linguistic abilities until global impairment of multiple cognitive functions occurs. β-Amyloid deposits are predominantly an aggregate of Aβ peptide, which in turn is a product of the proteolysis of amyloid precursor protein (APP) as part of the β-amyloidogenic pathway, Aβ peptide results from the cleavage of APP at the C-terminals by one or more γ-secretases and at the N-terminus by β-secretase enzyme (BACE1) also known as aspartyl protease 2. BACE1 activity is correlated directly to the generation of Aβ peptide from APP.
Studies indicate that the inhibition of BACE1 impedes the production of Aβ peptide. Further, BACE1 co-localizes with its substrate APP in Golgi and endocytic compartments (Willem M, et al. Semin. Cell Dev. Biol, 2009, 20, 175-182). Knock-out studies in mice have demonstrated the absence of amyloid peptide formation while the animals are healthy and fertile (Ohno M, et al. Neurobiol. Dis., 2007, 26, 134-145). Genetic ablation of BACE1 in APP-overexpressing mice has demonstrated absence of plaque formation, and the reversal of cognitive deficits (Ohno M, et al. Neuron; 2004, 41, 27-33). BACE1 levels are elevated in the brains of sporadic AD patients (Hampel and Shen, Scand. J. Clin. Lab. Invest. 2009, 69, 8-12).
These convergent findings indicate that the inhibition of BACE1 may be a therapeutic target for the treatment of AD as well as disorders for which the reduction of Aβ deposits is beneficial.
At the 2012 Alzheimer's Association International Conference in Vancouver, several drug developers announced their BACE1 inhibitors in clinical trials. Eli Lilly scientists reported preclinical research on LY2886721 which had entered a Phase II study. Merck presented a series of posters detailing the Phase I studies of its BACE inhibitor, MK-8931, and announced the start of separate Phase III studies which will test the compound for two years in people with prodromal Alzheimer's disease. Numerous patent applications directed to BACE1 inhibitors have been published over the past several years.
AstraZeneca announced the discovery of AZD3839, a potent and selected BACE1 inhibitor clinical candidate for the treatment of AD (Jeppsson, F., et al. JBC, 2012, 287, 41245-41257) in October 2012. The effort which led to the discovery of AZD3839 was further described in Ginman, T., et al. Journal of Medicinal Chemistry, 2013, 56, 4181-4205. The Ginman publication describes the issues which were overcome in connection with the discovery and identification of AZD3839. These issues related to poor blood brain barrier penetration and P-glycoprotein mediated efflux of the compounds resulting in lack of brain exposure.
The Ginman manuscript hypothesized that the differences would largely be due to the core structures and Structure Activity Relationship data was provided wherein the in vitro properties on the reported compounds were given into four tables according to core sub-types. In table 4, a series of amidine containing compounds are described that were considered interesting from an activity perspective. However, the data suggests that the amidine containing core did not exhibit a favourable blood brain barrier permeability profile.
Researchers from Hoffmann-La Roche and Siena Biotech also reported the discovery of amidine containing compounds (Woltering, T. J., et al. Bioorg. Med. Chem. Left. 2013, 23, 4239-4243). These compounds (compounds 17 and 18 in the paper) were found not to have any in vivo effect (lack of Aβ40 reduction in brain in wild type mice).